"I think it underscores the threat that al Qaeda in the Arab Peninsula poses to U.S. interests," President Obama's top counterterrorism adviser John Brennan told ABC's "This Week."

"We looked at the intelligence that is available as far as the plans for al Qaeda to carry out attacks in Sanaa possibly against our embassy, possibly against U.S. personnel, and decided it was the prudent thing to do to shut the embassy," Brennan said.

The British government joined the United States in closing its embassy in Yemen on Sunday, highlighting the Arab nation's emergence as one of the world's premier terrorist havens.

Yemen: New Front in War on Terror?

In the wake of the Christmas Day terror attempt, the president's deputy national security adviser for homeland security and counterterrorism reiterated the Obama's administration has been "investing in Yemen for many, many months."

"Al Qaeda has several hundred members in Yemen, and they've grown in strength," Brennan said. "From the very first day of this administration we've been focused on Yemen."

"Just this past month, we and the Yemenis were able to identify the location of some of these al Qaeda operatives and commanders and leaders, successful strikes that were carried out, and there were several of the al Qaeda members, operatives and the senior leaders who are no longer with us today as a result of those actions," Brennan said.

Congresswoman: Stop Sending Guantanamo Bay Detainees to Yemen

Rep. Jane Harman, chair of the House Homeland Security Intelligence Subcommittee, argued Guantanamo Bay detainees should not be sent back to Yemen in light of the country's struggle to contain al Qaeda.

"I believe the prison should close, but I also believe we should review again where we're going to send the detainees. I think it is a bad time to send the 90 or so Yemenis back to Yemen," Harman, D-Calif., said on "This Week."

The top Republican on the House Intelligence Committee argued on "This Week" that former Guantanamo Bay detainees are making up the "core group" of al Qaeda in the Arab Peninsula.

"This is an imminent threat that is coming from the Al Qaeda Arabian Peninsula area," said Rep. Peter Hoekstra, R-Mich., in his first interview since visiting Yemen.

"The core group of al Qaeda on the Arabian Peninsula is formed by former Gitmo detainees," Hoekstra said. "These are people that were held in Gitmo, have been returned and have now gone back to the battlefield."

Hoekstra said the other element posing a threat in Yemen is radical cleric Anwar al-Awlaki.

"So you've put the Gitmo folks together. You put Awlaki together. These people have moved an attack on the U.S. homeland to their -- to the top of their priority list. So that is the root cause of why we saw the attack at Fort Hood, why we saw the attack on the flight, Flight 253," Hoekstra said.